In this set of projects we propose to follow, using stable "markers," the physiological maturation of lymphocyte precursors in hematopoietic tissues, their migration to and maturation in the sites of T (thymus) and B cell generation; the migration of "mature" T and B lymphocyte classes to peripheral lymphoid tissues, including the cell membrane changes which accompany this maturation; and the antigen-dependent redistribution of T and B cells in the genesis and execution of cellular and humoral immune responses. The stable marker systems employed will be local labeling of cells with 3H-thymidine (3HTdR) or uridine (3HU) in situ, or the localized placement (in situ) of cells bearing distinctive cell membrane markers. The techniques for localizing, detecting, and separating descendants in distant sites include specific immunofluorescence of frozen tissue sections utilizing immune sera which distinguish specific cell membrane antigens, or fluorescence-activated cell sorting (FACS) of cell suspensions using these sera, or auto radiographic analysis of tissue sections and separated cell suspensions, or cell separation via velocity sedimentation. In an additional project we shall study the nature of thymocyte and thymic lymphoma cell surface moieties which effect binding of murine thymocyte and thymic lymphoma cell surface moieties which effect binding of murine leukemia virus (MuLV) virions.